A seedling node that handles clean shutdown of the JVM and of the Seedling
itself.

Nodes that need special shutdown handling can register themselves with
the ShutdownNode. This is cleaner and safer than using a standard shutdown
hook, since the handlers will be called before JVM shutdown if
the seedling is stopped programmatically.

startService

Starts a Seedling service. This is called by the Seedling runtime after
the node has been installed and all properties have been set to their
configured values.
Any exceptions thrown by an implementation of this method are handled
by the Seedling. Instances of RuntimeException are propagated
to the application, but any other kind of exception is wrapped in a
ServiceException and rethrown.